A conventional boiling water reactor includes a reactor pressure vessel within which is disposed a nuclear reactor core having a plurality of fuel bundles. The core is effective for generating heat released from nuclear fission reactions for boiling water contained in the vessel for generating steam to power a steam turbine for driving, for example, an electrical generator for providing power to a utility grid. The reactor core typically includes a plurality of control rods or blades containing solid nuclear poison which are selectively inserted and withdrawn therefrom by conventional control rod drives (CRDs) for controlling the nuclear reaction rate, or reactivity, within the core. A typical nuclear reactor includes a substantial number of control rods and corresponding control rod drives, for example, over 200 of each. The control rod drives are typically mounted externally of the vessel at either the upper or lower closure head of the vessel and have push rods which extend through the vessel and into the reactor core. Conventional CRDs either function hydraulically or electro-mechanically for positioning the control rods within the reactor core.
Fission reactors rely on fissioning of fissile atoms such as uranium isotopes (U.sup.233, U.sup.235) and plutonium isotopes (Pu.sup.239, Pu.sup.241). Upon absorption of a neutron, a fissile atom can disintegrate, yielding atoms of lower atomic weight and high kinetic energy along with several high-energy neutrons. The kinetic energy of the fission products is quickly dissipated as heat, which is the primary energy product of nuclear reactors. Some of the neutrons released during disintegration can be absorbed by other fissile atoms, causing a chain reaction of disintegration and heat generation. The fissile atoms in nuclear reactors are arranged so that the chain reaction can be self-sustaining.
To facilitate handling, fissile fuel is typically maintained in modular units. These units can be bundles of vertically extending fuel rods. Each rod has a cladding which encloses a stack of fissile fuel pellets. The bundles are arranged in a two-dimensional array in the reactor to form the core. The neutron-absorbing control rods are inserted between or within fuel bundles to control the reactivity of the core.
Both economic and safety considerations favor improved fuel utilization, which can mean less frequent refueling and less exposure to radiation from a reactor interior. In addition, improved fuel utilization generally implies more complete fuel "burnups", or fissioning.
A major obstacle to obtaining long fuel element lifetimes and complete fuel burnups is the inhomogeneities of the neutron flux both radially and axially throughout the core. For example, fuel bundles near the center of the core are surrounded by other fuel elements. Accordingly, the neutron flux at these central fuel bundles exceeds the neutron flux at peripheral fuel bundles which have one or more sides facing away from the rest of the fuel elements. Therefore, peripheral fuel bundles tend to burn up more slowly than do the more central fuel bundles.
The problem of flux density variations with radial core position has been addressed by repositioning fuel bundles between central and peripheral positions. This results in extended fuel bundle lifetimes at the expense of additional refueling operations.
Variations in neutron flux density occur in the axial direction as well as the radial direction. For example, fuel near the top or bottom of a fuel bundle is subjected to less neutron flux than is fuel located midway up a fuel bundle. These axial variations are not effectively addressed by radial redistribution of fuel elements.
In addition to the variations in neutron flux density, variations in neutron spectral distribution affect burnup. For example, in a BWR, neutrons released during fissioning move too quickly and have too high an energy to readily induce the further fissioning required to sustain a chain reaction. These high energy neutrons are known as "fast" neutrons. Slower, lower-energy neutrons, referred to as "thermal" neutrons, most readily induce fission.
In BWRs, thermal neutrons are formerly fast neutrons that have been slowed by moderation primarily through collisions with hydrogen atoms in the water (moderator) used as the heat transfer medium. Between the energy levels of thermal and fast neutrons are "epithermal" neutrons. Epithermal neutrons exceed the desired energy for inducing fission but promote resonance absorption by many actinide series isotopes, converting some "fertile" isotopes to "fissile" (fissionable) isotopes. For example, epithermal neutrons are effective at converting fertile U.sup.238 to fissile Pu.sup.239.
Within a core, the percentages of thermal, epithermal and fast neutrons vary over the axial extent of the core. Axial variations in neutron spectra are caused in part by variations in the density or void fraction of the water flowing up the core. In a BWR, water entering the bottom of the core is essentially completely in the liquid phase. Water flowing up through the core boils, so most of the volume of water exiting the top of the core is in the vapor phase, i.e., steam. Steam is less effective than liquid water as a neutron moderator due to the lower density of the vapor phase. Therefore, from the point of view of neutron moderation, core volumes occupied by steam are considered "voids"; the amount of steam at any spatial region in the core can be characterized by a "void fraction". Within a fuel bundle, the void fraction can vary from about zero at the base to about 0.7 near the top.
Continuing the example for the BWR, near the bottom of a fuel bundle, neutron generation and density are relatively low, but the percentage of thermal neutrons is high because of the moderation provided by the low void fraction water at that level. Higher up, neutron density reaches its maximum, while void fraction continues to climb. Thus, the density of thermal neutrons peaks somewhere near the lower-middle level of the bundle. Above this level, neutron density remains roughly stable while the percentages of epithermal and fast neutrons increase. Near the top of the bundle, neutron density decreases across the spectrum since there are no neutrons being generated just above the top of the bundle.
The inhomogeneities induced by this neutron spectral distribution can cause a variety of related problems. Focusing on the top middle-to-upper section, problems of inadequate burnup and increased production of high-level transuranic waste are of concern due to the predominance of fast and epithermal neutrons known as a "hard" neutron spectra, as opposed to a predominance of thermal neutrons known as a "soft" neutron spectra. Since this top section has a hard spectra with a relatively low percentage of thermal neutrons, a higher concentration (enrichment) of fissile fuel is sometimes used to support a chain reaction. If the fuel bundle has a uniform fissile fuel distribution, this section could fall below criticality (the level required to sustain a chain reaction) before the other bundle sections. The fuel bundle would have to be replaced long before the fissle fuel in all sections of the bundle were depleted, wasting fuel.
One conventional method of dealing with neutron spectral axial variations is to use control rods to selectively shape axial power distribution, and, as a consequence, the void distribution, for shifting the neutron spectral distribution. The neutron spectral distribution is shifted so that the core operates with a bottom peaked power distribution and high average void fraction promoting a hard neutron spectra during the early portions of the cycle in order to convert fertile to fissile fuel. Then, the neutron spectra are softened, or shifted, during the later portions of the cycle by operating with a top peaked power distribution and lower average void fraction to burn the additional fissile fuel so produced to extend the cycle.
For the BWR, control rods typically extend into the core from below and contain neutron-absorbing material (i.e. poison) which robs the adjacent fuel of thermal neutrons which would otherwise be available for fissioning. The fuel cycle between refueling can be conventionally extended by maintaining a small percentage (e.g. about 10%) of the control rods fully inserted into the core at the beginning of the cycle to promote a bottom peaked axial power distribution, with the majority being fully withdrawn during operation, i.e. generating power, and then continually withdrawing these shaping control rods during the cycle. In this way, a hard-to-soft neutron spectra distribution is initially effected from the top-to-bottom, respectively, of the fuel rods due to the resulting increasing void fraction from bottom-to-top of the fuel rods which also promotes power generation from the bottom of the core. As the shaping control rods are increasingly withdrawn from the core during the cycle, thier poisoning effect is correspondingly removed from the upper levels of the core thusly shifting upwardly the power distribution and softening the entire neutron spectral axial distribution due to reduction in the void fraction. Thus, control rods can be used to modify the distribution of thermal neutrons over axial position to achieve more complete burnups. However, control rods provide only a gross level of control over spectral distribution.
More precise compensation for spectral variations can be implemented using enrichment variation and burnable poisons. Enrichment variation using, for example, U.sup.235 enriched uranium, can be used near the top of a fuel bundle to partially compensate for a localized lack of thermal neutrons. Similarly, burnable poisons such as gadolinium oxide (Gd.sub.2 O.sub.3), can balance the exposure of bundle sections receiving a high thermal neutron flux. Over time, the burnable poisons are converted to isotopes which are not poisons so that more thermal neutrons become available for fissioning as the amount of fissile material decreases. In this way, fissioning can remain more constant over time in a section of the fuel bundle. By varying the amount of enrichment and burnable poisons by axial position along a bundle, longer and more complete burnup cycles can be achieved before refueling is required. In addition, the enrichment and poison profiles can be varied by radial position to compensate for radial variations in thermal neutron density.
Another conventional method used to affect neutron spectral axial distribution to extend fuel burn cycles is to initially decrease recirculation flow through the core to increase the average void fraction which, therefore, increases the neutron spectral hardness for converting fertile fuel to fissile fuel. Then later in the cycle, the recirculation flow can be increased to decrease average void fraction, and therefore soften the neutron spectra and burn the additional fissile fuel so produced.
Nonetheless, taken together, the use of control rods, recirculation flow control, radial positional exchange of bundles, selective enrichment and distribution of burnable poisons still leave problems with axial variations in burn rates and neutron spectra. Furthermore, none of these employed methods effectively address the problem of the high level of fissile material produced and left in the upper-middle sections of the bundle due to the high level of epithermal neutrons and the low level of thermal neutrons. What is needed is a system that deals more effectively with axial variations in neutron spectra so that higher fuel burnups are provided and so that high-level waste is minimized.